1. Field of the Invention
The subject invention pertains to the field of thermoplastic composites. More particularly, the subject invention pertains to a process for forming non-planar thermoplastic composites wherein drapeable thermoplastic prepregs are layed up and fused under a pressure differential in a thermoforming operation utilizing but a single diaphragm.
2. Description of the Related Art
Thermoplastic composites are becoming increasingly important items of commerce in the transportation and aerospace industries. Thermoplastic composites containing advanced engineering thermoplastics such as the polyarylene sulfides, polyimides, polyetherimides, polysulfones, polyetherketones and the like are superior in some respects over their thermosetting analogues due to their greater impact resistance and in some cases, greater solvent resistance. Unfortunately, the use of thermoplastic composites has been limited due to difficulties in fabrication of parts, especially parts of complex shape. This difficulty is due in large part to the nature of the thermoplastic impregnated prepreg itself, but also to the process by which layups containing these prepregs are fused to form an integral composite part.
In U.S. Pat. No. 4,770,838 for example, is disclosed a process of forming non-planar thermoplastic composite parts which has become standard in the industry. According to this process, a layup containing layers of unidirectional, woven, or random mat fiber-reinforcement which has been melt or solution impregnated with an engineering thermoplastic is sandwiched between two diaphragms of metal or high temperature plastic forming a very stiff, flat layup. This layup is then clamped into the mold and vacuum applied between the diaphragms with a vacuum ring assembly while heating to the melting temperature of the thermoplastic. Pressure is then applied to one side of the layup, and optionally, vacuum may be applied to the other side of the layup. As a result of the pressure and temperature, the entire layup is deformed into the shape of the mold surface.
Following cooling, the diaphragms are removed and of necessity discarded, as they have also been deformed to the shape of the part and cannot be reused to form a flat layup. As the diaphragm material is expensive, this process is quite wasteful. Moreover, the deformation of a flat layup into a complex part, particularly those with sharp corners, results in the partial separation of fibers, creating non-uniform cross-sections, resin rich and resin poor, or resin starved areas, and associated therewith, unpredictable physical properties.